The present invention relates to image taking devices such as a CMOS (Complementary Metal Oxide Semiconductor) image sensors and camera systems employing same.
In recent years, techniques for measuring minute amounts of light emitted by or from a biological body and fluorescent light, as well as techniques for taking images, have been developed in the fields of medical cares and biotechnologies.
In addition, a transmission image taking technology has been applied to products in the medical-care and security fields. In accordance with the transmission image taking technology, few X rays passing through an object of observation are converted by a scintillator into photons at the visible level and the photons are then detected in order to take an image of the object under observation.
In an image taking process carried out in such fields, a photon counter is used for measuring the minute amount of light.
Usually, the photon counter is a simple device which makes use of an avalanche diode or a photoelectron multiplying tube.
The device serving as the photon counter converts every photon incident on a light receiving surface of the device into a photoelectron and accelerates the photoelectron by the electric field thus multiplies the number of photoelectrons by, among others, secondary-electron generations due to collisions. Then, the device generates voltage pulses from the photoelectrons.
A counter apparatus connected at any point in time to the device serving as the photon counter counts the number of voltage pulses.
The photon counter has an excellent measurement precision which allows the photon counter to detect every photon. On the other side of the coin, however, a system employing the photon counter is expensive and the dynamic range of a measurement process is narrow.
Normally, the number of photo electrons that can be measured by a photon counter is about 1 million to 10 millions.
For an image taking process with a relatively large range of measured-light quantities, on the other hand, a photodiode and an AD (Analog to Digital) converter are used.
The photodiode is used for accumulating electric charge obtained as a result of an opto-electrical conversion process and for generating an analog signal. Then, the AD converter converts the analog signal into a digital signal.
However, the image taking process as described above raises problems. One of the problems is noises generated in propagation of the analog signal. Another problem is the speed of the AD converter.
In a process of detecting light having a small amount, it is necessary to reduce generated noises and increase the bit count of the AD conversion in order to improve the resolution of the digital signal. For this reason, an AD converter having an extremely high conversion speed is required. In addition, in order to improve the resolution of the taken image, it is necessary to increase the number of pixels of the image. Thus, the size of a system for AD conversion is very large.
For more information, refer to Japanese Patent Laid-open Nos. 1995-67043 and 2004-193675 (hereinafter referred to as Patent Documents 1 and 2, respectively).
In the nature of things, an operation to take an image emitting little light needs to be carried out by detecting the light with a high degree of precision by reduction of noises generated in the operation and also carried out at a large dynamic range.
However, there is no device that meets both of these requirements.
In an attempt to reduce the amount of contamination caused by an X-ray image taking operation for example, precision of the same level as a photon counter is required. By making use of an ordinary photon counter, however, a dynamic range sufficient for the image taking operation is not satisfied.
In addition, in order to improve the resolution, a multi-pixel system including a counter apparatus is demanded. However, such a system is extremely expensive.
As disclosed in Patent Document 1, on the other hand, there is proposed a new technique for counting the number of photons on a time-division basis.
In accordance with the technique, a binary value is produced to serve as a result of determination as to whether or not an incident photon has hit a photodiode in a time period determined in advance. The process to produce such a binary value is carried out for every photodiode and the values are integrated to give two-dimensional image taking data.
That is to say, a signal generated by a photodiode is sensed in each time period determined in advance and, if the number of photons incident on the photodiode as indicated by the signal during the time period is at least 1, a counter connected to a pixel corresponding to the photodiode is incremented by 1 regardless of the number of photons incident on the photodiode.
If the frequency of the photon incidence on the photodiode varies at random along the time axis, the number of actual incident photons and the contents of the counter follow the Poisson distribution. Thus, for a low frequency of the photon incidence, the relation between the number of actual incident photons and the contents of the counter is linear. Even if the frequency is high, the relation between the number of actual incident photons and the contents of the counter can be corrected in a uniform manner.
In accordance with the technology disclosed in Patent Document 1, however, every pixel requires a sense circuit and a counter so that the aperture area of the pixel inevitably becomes very small.
Patent Document 2 mentioned earlier proposes a configuration in which a counting technique based on time division is adopted and counters are provided outside an array of pixels which each require a sense circuit and a memory.
In addition, in spite of the fact that the counters can be provided outside the array of pixels, every pixel requires one of the counters. Thus, the circuit size of a chip including the counters is unavoidably large.
On top of that, in the configurations disclosed by Patent Documents 1 and 2, an attempt can be made to increase the dynamic range only by shortening the interval to measure incident photons along the time axis and by increasing the speed of accesses to the pixels.